DEPARTMENT OF EMBRYOLOGY. 121 



It has been found by Dr. C. R. Essick that, in the human embryo, 

 in the substance of the corpus striatum there is a separation of the 

 nervous tissues, causing the formation of two bilaterally symmetrical 

 transitory cavities — cavum mediale corporis striati and cavum laterahs 

 corporis striati. These cavities make their appearance in embryos 

 about 15 mm. long and disappear as the growth exceeds 20 mm. The 

 cavities vary in shape and may be either simple or multiple, but they 

 are always found in a constant position. An interesting feature is 

 the presence within the cavities of a large number of peculiar ameboid 

 phagocytic cells which otherwise are foreign to nervous tissue. They 

 resemble morphologically the macrophages of the adult and the Hof- 

 bauer cells of the embryonic chorion. The cavities are evidently of 

 functional significance, and the author advances the hypothesis that 

 they result from an accumulation of cerebro-spinal fluid and that they 

 disappear as soon as adequate drainage of their contents into the 

 meningeal spaces becomes estabUshed. 



Studies on different phases in the development of blood-vessels have 

 been published during the year by G. L. Streeter, E. R. Clark, and F. R. 

 Sabin. 



Dr. Streeter has made an embryological study of the venous sinuses 

 of the dura mater in the human embryo. This is in continuation of the 

 work published by Professor Mall in 1905. The present study is based 

 on human embryos belonging to the Department of Embryology and 

 in it is traced the development of the veins of the head in embryos from 

 4 mm. long upward, through successive stages to the adult conditions. 

 In addition to the value of the morphological details that are brought 

 out, it is of especial interest on account of its bearing on the broader 

 principles involved in the establishment of vascular drainage. The 

 particular group of veins studied affords striking examples of changes 

 and adjustments in drainage channels which are consequent upon the 

 alterations in the form and condition of the particular area drained. 

 The marked change in the form of the brain, and particularly the pro- 

 longed relative growth of the cerebral hemispheres, render necessary a 

 continuous series of alterations in the veins that extend throughout the 

 greater part of fetal life. 



To a great extent this adjustment is accomplished by the migration 

 of the principal blood-channels. This migration occurs in two ways. 

 There is the passive change in position or direction of the endothelial 

 tube itself, due to mechanical causes arising from alterations in its 

 environment. On the other hand, a vein may change its position by 

 forming or adopting a new endothelial channel, at the same time relin- 

 quishing its original endothelial channel. The plexiform arrangement 

 of the veins in the region of the tentorium is especially favorable for this 

 procedure, and in this region this type of alteration is repeatedly illus- 

 trated. In other words, under migration of veins one can distinguish 



